1. Technical Field
This disclosure relates to a liquid crystal display panel for displaying a video picture on a liquid crystal display unit, and to a liquid crystal display device provided with such a liquid crystal display panel.
2. Description of the Related Art
A liquid crystal display panel including a liquid crystal display unit which uses liquid crystals as the optical modulator comprises a backlight unit for illuminating the liquid crystal display unit from a rear face, and realizes the display of arbitrary images by changing the transmittance of the illumination light from the backlight unit with the respective liquid crystals.
As this kind of backlight unit, a liquid crystal module using a light-emitting diode (LED) as the light source is becoming widely prevalent. Today, the LED that is used as the backlight unit is often an LED which emits white light. A white LED usually obtains a white light by applying phosphors of yttrium, aluminum and garnet (YAG) or the like on a GaN-based blue LED chip, and synthesizing blue and yellow. Moreover, in recent years, the emission efficiency and color reproducibility are being improved by using a new phosphor or mixing phosphor which strongly emits red or green with the YAG phosphor. For example, Japanese Patent Application Publication No. 2008-218485 proposes a light-emitting device capable of maintaining high color rendering properties and changing the color temperature.
While an LED has a long service life, when it is used for several ten thousand hours, the light emission luminance decreases about several tens percent. With consumer equipment such as TVs and mobile phones, even if the luminance decreases, the equipment is used as is without correcting the luminance. However, with medical equipment and broadcast equipment, the decreases in luminance is unacceptable. Thus, normally, the luminance is stabilized by increasing the supply current to the LED for each given period of use or other methods. The LED has, in general, an upper limit value (allowable current) of the current that can be supplied. Accordingly, for instance, a method is adopted where, at the beginning of use, light is emitted at a current value that is ½ of the allowable current, and the supply current is increased according to the decrease in the light emission luminance. Otherwise, a method is adopted where, at the beginning of use, the allowable current is supplied at 50% of the lighting duty ratio, and the lighting duty ratio is increased according to the decrease of the light emission luminance.
The luminance of an LED can be stabilized, without causing the gradation to deteriorate, by increasing the supply current to the LED or increasing the lighting duty ratio as described above. On the other hand, with an LED, there are cases where the chromaticity changes, similar to the luminance, due to a temporal change. Similar to the case of luminance, with consumer equipment such as TVs and mobile phones, even if the chromaticity changes, the equipment is used as is without correcting the chromaticity. However, with medical equipment and broadcast equipment, the change in chromaticity is unacceptable as with the decrease in luminance, and in particular the change in the chromaticity point of white is unacceptable. Thus, it is necessary to inhibit changes in the chromaticity. However, the foregoing Japanese Patent Application Publication No. 2008-218485 does not in any way describe the inhibition of changes to the chromaticity caused by a temporal change of the LED.